Apparatus for improved liquidvapor contact



Nov. 1, 1966 w. BRUCKERT ETAL 3,232,576

APPARATUS FOR IMPROVED LIQUID-VAPOR CONTACT 2 Sheets-Sheet 1 OriginalFiled March 2, 1965 INVENTORS WALTER BRUCKERT DAVID l-J. WANG 064.

A TTORNEV Nov. 1, 1966 w. BRUCKERT ETAL 3,282,576

APPARATUS FOR IMPROVED LIQUID-VAPOR CONTACT Original Filed March 2, 19652 Sheets-Sheet 2 INVENTORS WALTER BRUCKERT DAVID l-J. WANG ATTORNEYUnited States Patent O 3,282,576 APPARATUS non IMPROVED LIQUID- VAPORCONTACT Walter Bruckert, Feldstrasse, Germany, and David 1-3..

Wang, Buifalo, N.Y., assignors to Union Carbide Corporation, acorporation of New York Continuation of application Ser. No. 436,430,Mar. 2, 1965. This application Jan. 28, 1966, Ser. No. 532,040 6 Claims.(Cl. 261114) This is a continuation of application Serial No. 436,430,filed March 2, 1965, now abandoned, which in turn is acontinuation-in-part of application Serial No. 136,796, filed September8, 1961, and now abandoned.

The present invention relates to an improved liquidvapor contact trayand in particular to means for promoting complete tray activity over theentire tray sur' face.

Typically, in mass transfer operations employing plates and trays, asfor example in distillation, a descending liquid is brought intointimate contact with a rising vapor by repetitive intermixing anddisengagement of the two phases. The degree of mass transfer of a givencomponent between these phases is determined largely by the intimacy ofthis contact. Maximum utilization of each tray requires close approachto phase equilibrium at all points on the tray, but this condition hasrarely been ob tained by the prior art.

A major factor which has prevented reasonable approaches to phaseequilibrium has been the existence of partial inactivity of thecontacting surface whereby the process vapor fails to pass through theprior art tray and the liquid thereupon at all points. Thus asubstantial portion of the contacting surface merely'transports clearliquid and contributes nothing to the mass-transfer process.Furthermore, such inactive areas are prone to weep or drain liquid tothe tray below thereby altering liquidvapor ratios on one tray andupsetting fluid compositions on the next.

Partial inactivity occurs on a given tray when the tray and its liquidpresent non-uniform resistance to vapor flow entering the tray. A commoncause is the hydro static gradient which normally comprises the driving.force for moving the liquid across the tray. Inactivity can be reducedsignificantly by eliminating this gradient, for example, by employmentof vapor thrust, rather than gravity, to propel the liquid. Vapor thrustfor this purpose may be generated by inclined openings properly arrangedto move the liquid at desired velocity and in a desired direction.

Despite the elimination of gradient and other improvements we have notedthat a certain area, notably the liquid inlet area, exhibits chronicinactivity and have discovered means to impart full activity to sucharea. Furthermore, this activation is accomplished without resort to theusual, expensive remedy of increasing the vapor phase pressure drop byreducing the free area of the tray.

As used herein the term clear liquid refers to a single phase, liquidfluid without physical admixture with process vapor. The term is notrestricted to liquids having optical clarity, but include liquidsrendered cloudy or opaque by dispersion of substances therein other thanprocess vapor as well as those liquids which are naturally cloudy oropaque.

The terms bubbling liquid, foam, froth and active liquid refer to aliquid through which a gas or vapor is being passed.

The term tray proper refers to that portion of a tray surface which isperforated or apertured for vapor flow, exclusive of that portionillustrated and discussed hereinafter as the bubble promoter area.

It is the prime objective of this invention to provide means to reducethe pressure drop across partially active liquid-vapor contact trayswhile inducing substantially complete tray activity.

It is another object of this invention to provide means for eliminatingthe inactive liquid-inlet area on liquidvapor contact trays, notablyperforated or sieve trays, wh'en thee liquid is induced to flow acrossthe tray by a liquid gradient.

It is a further objective of this invention to provide means foreliminating the inactive liquid-inlet are on liquid-vapor contact trays,notably perforated orsieve trays when the liquid is induced to flowacross the tray by vapor thrust.

It is a still further objective of this invention to provide processwhereby partially active liquid-vapor contact trays are made completelyactive.

It is a still further objective of this invention to provide means,which, when used in conjunction with liquidvapor contacting trays willrender partially active trays completely active.

These and other objects and advantages of the instant invention will beapparent from the description and appended claims.

According to the present invention the liquid inlet of a given tray isconstructed in such a manner as to appreciably reduce the hydrostatichead in this area preferably to a value actually less than that existingdownstream of the liquid inlet. This reduced or artificial hydrostatichead presents to the rising process vapor a lower flow resistance at thetray inlet and improves the activity level thereat.

It has been found that bubbling on liquid-vapor contact trays is muchmore diflicult to initiate than to maintain. This is because at twoneighboring points on a tray the liquid, whether aerated or clear, tendsto maintain the same potential energy. However, at equal potentialenergy an aerated :body of liquid exerts a smaller hydrostatic head thana clear body of liquid. Therefore, once aeration or bubbling isinitiated it tends to persist since the resistance to flow of the vaporphase is thereafter lower. Because of this difficulty in initiatingbubbling it is preferable that the hydrostatic head at the inlet be lessthan that on the tray proper.

Once the tray inlet area has been rendered active by the bubblingpromoter of this invention, i.e., once process vapor is caused to flowthrough the liquid immediately entering the tray, the fluids flowingacross the tray will induce conditions favorable to bubbling throughoutthe tray proper. With an active inlet insured, it is found that the trayproper can be maintained fully active with far less mechanical pressuredrop through the tray than heretobefore possible.

The ability of the bubble promoters of this invention to render theinlet area of the tray active, which in turn induces activity throughoutthe tray, is of major significance. Various design and operatingparameters are markedly and beneficially changed by this invention. Thenew parameter changes allow substantial savings in the power required tomove the process vapor through the mass transfer apparatus, e.g., adistillation column. FIG. 1 is a view in cross-sectional elevation of atray illustrating the sloped-inlet embodiment of this invention as thebubble promoter embodied in a sleeve type tray;

FIG. 2 is a cross-sectional elevation view of a tray illustrating thesloped inlet embodiment of this invention as the bubble promoterembodied in a slotted sieve type tray;

FIG. 3 is an isometric View of a portion of a slotted sieve trayillustrating the sloped inlet embodiment of this invention as thebubblepromoter.

Patented Nov. l, 1966 By this invention complete tray activity isinsured by reducing the total resistance to vapor passage at the liquidinlet of an operating tray. The liquid inlet is constructed so as toartificially reduce the hydrostatic head of liquid at the tray inletpreferably to a value lower than the hydrostatic head existing on thetray proper. This permits utilization of a phenomenon that will bereferred to as dynamic enhancement. This phenomenon is a condition ofinduced activity extending over the entire surface of the tray properdownstream from a perpetually active inlet. It results from the flow orspread of low density froth from an active region across areasdownstream thereof. Dynamic enhancement requires that the active liquidWashing effect must be available to all areas on the tray proper at alltimes; the onset of inactivity must never occur at the tray inletotherwise the effect will be lost. The bubble promoters of thisinvention are needed to insure activity at the inlet. Prior art trayswould not benefit from dynamic enhancement because the inlet area hasbeen the most prone to become inactive.

In large distillation columns where the number of trays becomes quitelarge a substantial power savings is eralized when the bubble promotersof this invention causing dynamic enhancement and permitting dry platepressure drop reduction are employed. The results presented in Table Aillustrate this point. The values of column pressure savings aspresented in Table A, are based upon liquid air separation operations inthe uppercolumn of a standard double-column air fractionater having 45trays in the upper column. The inclined bubble promoter, as illustratedin FIGS. 1, 2 and 3 is used as the means for achieving full trayactivity in those cases where a promoter is used.

tower 130 forms a downcomer 14-8 for the passage of liquid downwardlyfrom the liquid discharge end 150 of contact tray 1111 to the liquidapproach region 152 of tray 1338. Adjacent the lower portions ofdowncomers 132 and 148 are seal pans 136 having bases 154 and obtuselysloped walls 156. The vertical, tray-side portions of downcomer elements146, 158 and obtusely sloped Walls 156 of seal pans 136 forming liquidapproach regions 152. Communicating with obtusely sloped walls 156 aresloped bubble promoters means 161) which extends transversely acrosstrays 110 and 138 and all other trays within column 1311. Bubblepromoter 169 comprises a flat, sloped surface 161 which may be integralwith or attached by some suitable means to the contact ing surface 162of tray 1111. Sloped bubble promoter 169 has perforations 118a definedby Walls which are substantially normal to the solped surface ofpromoter 160, however, these walls may be inclined to the surface ofpromoter 161) such that the angle of inclination would be in the firstquadrant. Any attempt to incline the walls of perforations 118a suchthat the angle of inclination of the walls would lie in the secondquadrant would cause a force to be exerted up the promoter surfacerather than down the surface. Such a force would tend to cause theliquid to back up in the liquid inlet thus destroying the usefulness ofthis invention. It would be possible to have perforations orientated indilferent direction, i.e., some directing vapor up the surface and somedirecting vapor down the surface, but in such a distribution such thatthe net force would be down the surface of promoter 160. Still anotherpossibility would be to orient all the perforations such that each hadan angle of inclination in the first quadrant. The choice of the angle,however,

TABLE A.-COMPARI'SON OF SIEVE TRAYS WITH AND WITHOUT BUBBLING PROMOIERSAs can be seen from Table A, the use of bubble promoters with each typeof tray, cases 2 and 4 compared with cases 1 and 3 respectively,markedly decreases the gas phase pressure drop and hence decreases thepower cost associated with moving the process vapor through the column.It is also significant that the power savings realized in case 4, vaporthrust apertures with promoter, is greater than the sum of that due tovapor thrust, apertures, case 3, and promoter used separately, case 2.

Referring to FIG. 1, there is shown a conventional fractionating columnor tower 130 having a plurality of perforated or sieve-type liquid-gascontacting trays therein and arranged one over the other. One such tray110 is shown in its entirety. Also shown is a downcorner 132 of a trayabove 134, a seal pan 136 and a portion of a sieve tray 138 below areshown. These trays define a plurality of vertically spaced liquid-gascontact stages 140 and 142 through which vapors pass upwardly in thetower 130. Each of the trays 110, 134 and 138 is supported and securedto the walls of tower 130 by tray holder 144. All of the trays withinthe tower 130 for a particular illustration will have the same generalfeatures of construction, and though the discussion hereinafter will bedirected more specifically to tray 110, it should be clear that it isequally applicable throughout the tower. Extending below tray 110 is adowncomer element 146 which, in cooperation with the side wall of wouldpresent some difficulty in that if too many perforations were inclinedtoo steeply the depth of liquid flowing over the surface of promoterwould become too thin resulting in blowing or fluidization of theliquid.

Clear process liquid descending from the tray-above 134 is directed bydowncomer baffie element 146 into downcomer 132 and into the base 154 ofseal pan 136. The liquid thereupon ascends obtusely sloped wall 156 andenters liquids approach region 152. When the clear liquid reaches thetop of obtusely sloped wall 156, thereby exiting from liquid approachregion 152, the liquid flows down bubble promoter 160 and onto thecontacting surface 162 of the tray proper. As the liquid flows downpromoter surface 161 it flows over perforations 118a which have processvapor passing nherethrough. The clear liquid entering bubble promoter160 presents to the rising process vapor an artificial head of liquidlower than that head of liquid which is on the contacting surface 162 oftray 116. As a result of this artificial head, which manifests itself asa reduced pressure region, the clear liquid emerging from liquidapproach region 152 is immediately transformed into an active liquid orfroth. Vapor flowing through the liquid in this region thereby makes theinlet portion of the tray completely active. As a result of the clearliquid emerging from region 152 being made active, dynamic enhancementcarries and sustains this activity over the tray proper therebyrendering the entire tray surface active.

The percent free area in the form of perforations will varyconsiderably, depending upon the physical properties of the liquids andvapors of any liquid-vapor contacting operation. The percent free area,however, should not be so great as to cause weeping of tray liquidthrough the perforations at reasonable liquid and vapor loadings. Whenutilizing a sloped bubble promoter, such as 160 in FIG. 1, it ispreferable to match the perforated area of the sloped bubble promotersurface to the perforated area of the contacting surface 162, i.e., thepercent free area of the bubble promoter surface is approximately equalto the percent free area of the contacting surface. For sloped bubblepromoters used in conjunction with perforated trays having vapor thrustopenings in addition to perforation, such as FIG. 3, it is preferable tohave the free area of thepromoter between the percent free arearepresented by perforations plus the percent free area represented byvapor thrust openings of the contacting surface.

Table B presented below, demonstrates the ability of the sloped bubblepromoter of FIG. 1 to render active trays that under similar flowconditions were partially inactive.

The data of Table B is for the system air-water. The experiments wereconducted using a perforated or sieve type tray 2 feet long and 1 footwide and having perforation 0.1875 inch in diameter. The perforated arearepresented approximately 11% of the total tray area. The gas and liquidflows in each experiment were chosen such that when used with the testtray without sloped bubble promoter, the first eight inches of the testtray were inactive, that is, 37.5% of the total tray surface wasinactive under the same liquid and gas flows in the absence of a bubblepromoter.

TABLE B.EXPERIMENTAL EVALUATION OF SLOPED INLET BUBBLE PROMOTER howQL/b, Vs, Remarks ft. /sec./ft. ft./sec.

1% 0. 05 1. 85 Fully active.

1% 0. l 2. 60 Do.

0 0.16 3. 10 Fully active, turbulent at inlet.

Where; how=height of outlet weir. inches.

Q1 lb=Cubic feet of tray liqnid/secJfoot of tray width. V.=Superficialvapor velocity, ftJse-c.

Bubble promoter dimensions: 6 inch high x 2 in. base; 14 /2" angle, seeFIG. 3.

Referring now to FIG. 2, there is shown a conventional fractionatingcolumn or tower 130 having a plurality of perforated or sieve-typeliquid-gas contacting trays hav ing vapor thrust openings 113 or slotsfrom which process vapor flowing therethrough contacts a process liquidflowing across at some angle thereby causing the liquid to flow acrossthe tray without the aid of liquid gradients. Thrust openings 113 arearranged in parallel rows on the tray surface and generally face theliquid outlet 150 of the tray, and are formed, as previously discussed,by the cooperation of vapor flow directing surface 123 with contactingsurface 162. Tray elements similar to those previously discussed will begiven the same numerical designation in the interest of clarity.Liquid-vapor contact trays, represented by tray 110, are arranged incolumn or tower 130, one over the other such that the trays withincolumn 130 form liquid-vapor contact stages represented by 140 and 142.Each tray is equipped with a downcomer element 146 which cooperates withthe walls of tower to form a downcomer represented by 132. Each tray issecured to the tower wall by tray holder 144. All of the trays withincolumn 130 will have the same general features of construction as thatshown by tray 110. Extending below tray 134, located above tray 110, isdowncomer element 146 which, as previously mentioned, forms downcomer132 for the passage of liquid downwardly from the tray above 134 to theliquid approach region 152 of tray 110. Adjacent the lower portion ofdowncomer 132 is seal pan 136 having base 154 and obtusely sloped wall156. The vertical, tray-side portion 158 of downcomer element 146 formswith base 154 and obtusely sloped wall 156 a liquid approach region 152.Communicating with obtusely sloped wall 156 is sloped bubble promotermeans 160 which extends transversely across tray 110. Bubble promoter160 being comprised of a fiat, sloped surface 161 which may be integralwith or attached by some suitable means to the contacting surface 162of-tray 110 and has perforations 118a defined by walls which aresubstantially normal to the sloped surface 161 of promoter 160. Bubblepromoter 160 may also be integral with or attached by some suitablemeans to obtusely sloped wall 156 of seal pan 136. It should be notedthat bubble promoter 160 is not equipped with vapor thrust openings 113as is tray 110.

Process liquid descending from tray-above 134 is directed by downcomerelement 146 into downcomer 132 and onto the base 154 of seal pan 136.The liquid thereupon ascends obtusely sloped wall 156 and enters liquidapproach region 152. When the liquid reaches the top of obtusely slopedwall 156, thereby exiting from liquid approach region 152, the liquidflows down promoter 160 and onto the contacting surface 162 of tray 110.As the liquid flows down promoter surfaces 161 it flows overperforations 118a which have process vapor passing therethrough. Theliquid entering bubble promoter 160 presents to the rising process vaporan artificial head of liquid lower than that which is on contactingsurface 162 of tray 110. As a result of this artificial head, the clearliquid emerging from liquid approach region 152 is immediatelytransformed into an active liquid or froth as a result of the reducedhead of liquid and vapor flowing therethrough thereby making thisportion of the tray completely active. As a result of the clear liquidfrom region 152 being made active, the activity is swept across the trayproper thereby rendering the entire tray surface active.

The data of Table C were obtained under operating conditions in afractionating column of an air separation plant. Test trays 25.5" inlength by 12" width were employed with and without the sloped bubblepromoter of this invention. The test trays were of the vapor thrustconstruction as shown in FIG. 2. The perforations normal to the traysurface had 0.036 inch diameters. The vapor thrust openings or slots hadan average density on the tray surface of 2.3 slots/in. and each hadapproximate dimensions; length 0.186", height 0.025. The combined freearea of the perforations and vapor thrust openings representedapproximately 14% of the total area.

TABLE C. COMPARISON OF OPERATING TRAYS WITH AND WITHOUT BUBBLE PROMOTERSNo Bubbling Sloped Inlet Promoter Promoter Vs, QLv t'tJsec. c.f.s.

Foam Area Foam Area Depth, Inactive, Depth, Inactive. Inches PercentInches Percent A comparison of the results apeparing in Table C showthat the slope bubble promoter completely eliminates tray inactivity atliquid and vapor loads which theretofore made the same tray partiallyinactive.

FIG. 3 is an isometric view of a portion of the sloped bubble promoterof this invention illustrating the association of promoter 160 withsieve tray 119 having vapor thrust openings 113. FIG. 3 isrepresentative of the bubble promoter used to gather the operating dataof Table C. The dimensions of bubble promoter 16!), a and b of FIG. 2,may vary considerably depending upon the properties of the liquid andvapor present in a given mass transfer operation. The b dimension ofpromoter 169 is the controlling dimension. We have found that values ofb may vary from just above zero to not greater than about 1.0 inch. Wesay just above zero because it has been discovered that any reduction inthe liquid height as it enters upon the contacting surface is beneficialto bubble promotion or dynamic enhancement. As a practical matter,however, values, of b from about A to 1.0 inch would be sufiicient toachieve the objects of this invention when the slope angle 115 isbetween about /5 to Above 1.0 inch the depth of liquid at the highestlocation on promoter 160, at reasonable liquid loads, becomes too thinand blowing or fiuidization of the clear liquid results as the clearliquid descends bubble promoter 160. A further restriction on the bdimension comes about from the fact that excessive downcomer heat lossesresult if b l.0 inch. If the slope values are below /5 the a dimensionbecomes excessive and a substantial portion of the tray becomes slopedcreating severe fabricating difificulties; if the slope values exceedthe inclination of the promoter becomes too steep and causes the liquiddepth to run thin on the promoter which causes a result similar to thatwhich occurs when b=1.0, viz., blowing or fiuidization of the clearliquid occurs.

The use of bubble promoting devices, such as those illustrated herein,are not limited solely to the types of trays which have been illustratedand discussed. A particular shape, form or type of liquid-vaporcontacting device is not essential to the successful use of bubblepromoters of this invention. The criteria which must be met forsuccessful use of the bubble promoters herein discussed with aliquid-vapor contacting device are that the mass transfer operation tobe conducted on the particular device be such that a clear processliquid is to be transformed into a froth by passing a process vaportherethrough. Thus, in addition to those devices illustrated herein,pitched, corrugated, bubble cap, rippled, circular flow and many othercontacting trays could successfully employ the novel promoters hereindescribed.

Although preferred embodiments of the invention have been described indetail, it is contemplated that some modifications thereto may beemployed without others, all within the spirit and scope of thisinvention as hereinabove set forth.

What is claimed is:

1. A liquid-gas contacting tray for use in a distillation tower whichcomprises:

(a) an imperforate inlet surface forming part of the perimeter of suchtray, adapted to be secured normal to the inner wall of saiddistillation tower for receiving liquid;

(b) a downcomer member adapted to be positioned substantially parallelto the distillation tower inner wall, spaced above and coextensive withthe inlet surface (a) and inwardly from the tray perimeter so as to forman exit path beneath the lower end of said downcomer member for saidliquid from such inlet surface;

(0) a wall contiguously associated and coextensive with the innermostedge of inlet surface (a) and extend ng upwardly from such edge beinginwardly positioned from .dQWIlCQmer member (b) so as to constitute aliquid approach path communicating with said exit path; 7

(d) a downwardly and inwardly inclined wall contiguously associated andcoextensive with the upper edge of wall (c) having perforations acrossthe entire surface thereof and of sufficiently small crosssectional areafor only upward gas flow therethrough, with the perforation walls beinginclined to the top surface of such inclined perforated wall (d) so thatthe angle of inclination with respect to the downhill side of said topsurface is in the first quadrant, thereby permitting the bubbling ofrising gas into the downwardly flowing liquid of reduced depth;

(e) a liquid-gas contact member having a first edge contiguouslyassociated and coextensive with the lower edge of downwardly andinwardly inclined perforated wall ((1) constructed to extendtransversely across at least part of the distillation towercross-section, inwardly from inlet surface (a), downcomer member (b),wall (c), and downwardly inclined perforated wall (d), having amultiplicity of openings therethrough for gas flow from the bottom sideto the top side and bubbling through the already bubbling liquid-gasmixture flowing across the top side thereof from the downwardly andinwardly inclined perforated wall (d); and

(f) a liquid discharge opening at the perimeter of said tray oppositefrom said imperforate inlet surface (a) being contiguously associatedwith a second edge of said liquid-gas contact member (e) opposite to thefirst edge of such member.

2. A liquid-gas contacting tray according to claim 1 in which saidliquid-gas contact member (e) is flat and has a plurality of fixedopenings extending therethrough.

3. A liquid-gas contacting tray according to claim 1 in which saidliquid-gas contact member (e) has a main flat surface with a pluralityof fixed first openings extending therethrough and defined by wallssubstantially normal to said main fiat surface; and a plurality of fixedsecond slot openings each of larger cross-sectional area than each ofsaid first openings and extending through said surface, each slotopening formed by a section raised from said main flat surface beingjoined thereto by an inclined back wall and having a front leading edgeseparated from and above such surface and oriented to face said liquiddischarge opening (f);

4. A liquid-gas contacting tray according to claim 1 in which saidliquid-gas contact member (e) has a main flat surface with a pluralityof fixed first openings extending therethrough and defined by wallssubstantially normal to said main fiat surface; and a plurality of fixedsecond slot openings each of larger cross-sectional area than each ofsaid first openings, each slot opening formed by a section raised fromsaid main flat surface being joined thereto by two inclined side wallsand an inclined back wall with the upper edge of each well joined to araised cover, the front leading edges of said side walls and said coverbeing separated from and above said main flat surface and oriented toface said liquid discharge opening (f).

5. A liquid-gas contacting tray according to claim 1 in which said angleof inclination of said perforation walls with respect to the downhillside of said top surface of downwardly and inwardly inclined wall (d) isdegrees.

6. A liquid-gas contacting tray according to claim 1 in which wall (c)is inclined with respect to imperforate inlet surface (a).

References Cited by the Examiner UNITED STATES PATENTS 2,693,949 11/1954Huggins. 2,702,696 2/1955 Pappas.

2,752,139 6/1956 Huggins 261-414 (Other references on following page)UNITED 9 STATES PATENTS Huggins et a1. Gilmore. Maille.

Thrift.

10 FOREIGN PATENTS 316,719 12/1956 Switzerland.

HARRY B. THORNTON, Primary Examiner.

5 RONALD R. WEAVER, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,282,576 November 1, 1966 Walter Bruckert et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 8, for "thee" read the line 11, for "are" read areacolumn 3, line 23, for era1ized" read realized column 4, line 57, for"liquids" read liquid column 7, line 1, for "apeparing" read appearingline 29, for "heat" read head a Signed and sealed this 5th day ofSeptember 1967.

(SEAL) Attcst:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Offioer Commissioner ofPatents

1. A LIQUID-GAS CONTACTING TRAY FOR USE IN A DISTILLATION TOWER WHICHCOMPRISES: (A) AN IMPERFORATE INLET SURFACE FORMING PART OF THEPERIMETER OF SUCH TRAY, ADAPTED TO BE SECURED NORMAL TO THE INNER WALLOF SAID DISTILLATION TOWER FOR RECEIVING LIQUID; (B) A DOWNCOMER MEMBERADAPTED TO BE POSITIONED SUBSTANTIALLY PARALLEL TO THE DISTILLATIONTOWER INNER WALL, SPACED ABOVE AND COEXTENSIVE WITH THE INLET SURFACE(A) AND INWARDLY FROM THE TRAY PERIMETER SO AS TO FORM AN EXIT PATHBENEATH THE LOWER END OF SAID DOWNCOMER MEMBER FOR SAID LIQUID FROM SUCHINLET SURFACE; (C) A WALL CONTIGUOUSLY ASSOCIATED AND COEXTENSIVE WITHTHE INNERMOST EDGE OF INLET SURFACE (A) AND EXTENDING UPWARDLY FROM SUCHEDGE BEING INWARDLY POSITIONED FROM DOWNCOMER MEMBER (B) SO AS TOCONSTITUTE A LIQUID APPROACH PATH COMMUNICATING WITH SAID EXIT PATH; (D)A DOWNWARDLY AND INWARDLY INCLINED WALL CONTIGUOUSLY ASSOCIATED ANDCOEXTENSIVE WITH THE UPPER EDGE OF WALL (C) HAVING PERFORATIONS ACROSSTHE ENTIRE SURFACE THEREOF AND OF SUFFICIENTLY SMALL CROSSSECTIONAL AREAFOR ONLY UPWARD GAS FLOW THERETHROUGH, WITH THE PERFORATIONS WALLS BEINGINCLINED TO THE TOP SURFACE OF SUCH INCLINED PERFORATED WALL (D) SO THATTHE ANGLE OF INCLINATION WITH RESPECT TO THE DOWNHILL SIDE OF SAID TOPSURFACE IS IN THE FIRST QUADRANT, THEREBY PERMITTING THE BUBBLING OFRISING GAS INTO THE DOWNWARDLY FLOWING LIQUID OF REDUCED DEPTH;